Cannabis use 'will impair but not damage mental health'

Regular cannabis use can have "real and significant" mental health effects but is unlikely to cause schizophrenia, according to a report from Government drugs advisers published yesterday.

The drug can impair psychological and psychomotor performance, cause acute intoxication reactions and lead to relapses of individuals with mental illnesses.

But the Advisory Council on the Misuse of Drugs said that on current evidence smoking cannabis was likely to increase the chances of developing schizophrenia by just one per cent.

The council, which was asked to reconsider the Government's decision to downgrade cannabis from a Class B to a Class C substance, recommended that it should not be reversed.

It had been asked by ministers to look afresh at medical evidence suggesting that more powerfully psychoactive varieties of the drug were posing an increased danger to mental health.

But the committee concluded: "For individuals, the current evidence suggests, at worst, that using cannabis increases the lifetime risk of developing schizophrenia by one per cent.

Some individuals are at higher risk than others for developing schizophrenia from the use of cannabis, but there is currently no means by which these individuals can be identified.

"The evidence for the existence of an association between frequency of cannabis use and the development of psychosis is, on the available evidence, weak. The council does not advise the reclassification of cannabis products to Class B; it recommends they remain within Class C.

"While cannabis can, unquestionably, produce harms, these are not of the same order as those of substances within Class B."

The council said that since it recommended in 2003 that cannabis should be downgraded, "further evidence has emerged about the possible link between the use of cannabis and the subsequent development of psychotic symptoms.

"While these studies do not of themselves prove beyond reasonable doubt that such a link exists, the accumulating evidence suggests that there is a causal association.

However, the consumption of cannabis is neither a necessary, nor a sufficient, cause for the development of schizophrenia.

"In the last year, over three million people appear to have used cannabis but very few will ever develop this distressing and disabling condition. And many people who develop schizophrenia have never consumed cannabis.

Based on the available data the use of cannabis makes (at worst) only a small contribution to an individual's risk for developing schizophrenia."

However, the council emphasised that cannabis use was harmful - it can also cause bronchitis and cancer - and should be discouraged. To that end, it wanted to see "a sustained education and information strategy" and more research into the links with mental health problems.

In the Commons yesterday, Charles Clarke, the Home Secretary, confirmed that he proposed to keep cannabis as a Class C drug, which means police generally take a more lenient line with personal possession and penalties are lower.

He said guidelines to police setting out the amount of cannabis that would be assumed to be for personal consumption would be lower than the four ounces proposed in a consultation paper last year. Such an amount would be enough to roll about 512 light joints or about 256 strong ones.

Mr Clarke said that although he was not proposing to reclassify cannabis, the message had to go out that it was harmful and that "its use can lead to a wide range of physical and psychological hazards".

He ordered a review of the classifications system which dates to 1971 and which critics say is confusing and misleading. Mr Clarke also asked the council to look again at the classification of the so-called "date-rape" drugs Rohypnol and GHB, which are currently Class C substances.

David Davis, the shadow home secretary, condemned the decision to reclassify cannabis as a confused message that would lead some "to continue thinking cannabis is a safe, 'soft' drug".

However, Mark Oaten, the Liberal Democrat spokesman, said: "Charles Clarke is right to base his decision on the best available evidence, and not on hysteria or political pressure. Cannabis is not harmless, but it is less harmful than many other illegal drugs."